a) Field of the Invention
The present invention relates to a liquid crystal display device having the characteristics that light transmission changes with an angle of incidence of light.
b) Description of the Related Art
A liquid crystal display device controls orientation of liquid crystal molecules to achieve a desired display. An orientation method such as rubbing is used to orient liquid crystal molecules in a desired direction under the conditions that a drive voltage is not applied to the liquid crystal layer. For this rubbing, an alignment film such as polyimide is coated on a substrate, and the surface of the film is rubbed with a rubbing cloth or the like in a predetermined direction.
Liquid crystal molecules are generally oriented in the rubbing direction and have pre-tilt raising their easy axes on the side of the rubbing end point from the substrate surface relative to the rubbing start side.
The rubbing direction can therefore be dealt with as a vector amount. The direction of a vector is defined, for example, as a direction from the rubbing start point toward the rubbing end point.
A typical orientation structure of a liquid crystal layer is a twisted nematic structure. Rubbing is performed usually in two crossing directions on a pair of substrates sandwiching the liquid crystal layer. Liquid crystal molecules have a predetermined twist such as 90.degree. from one substrate toward the other substrate in the plane parallel to the substrate surfaces.
If a pair of polarizers having crossed absorption axes (or transmission axes) is mounted outside of the liquid crystal layer, the state without an applied voltage becomes a light transmission state (on), and the state with an applied voltage becomes a light shielding state (off). This is called normally-on (normally-white).
If a pair of polarizers having parallel absorption axes (or transmission axes) is used, the state without an applied voltage becomes a light shielding state (off), and the state with an applied voltage becomes a light transmission state (on). This is called normally-off (normally-black).
As a method of displaying an image by using a liquid crystal layer, a direct view type and a projection type are known. With the former, a viewer sees an image directly at the liquid crystal layer, and with the latter, a viewer sees an image formed by projecting onto a screen light passed through the liquid crystal layer.
FIGS. 9A and 9B are a schematic diagram and a graph illustrating a projection type twisted nematic liquid crystal display device according to prior art. In FIG. 9A, a pair of glass substrates 101 and 102 sandwiching a liquid crystal layer 103 constitutes a liquid crystal display element (liquid crystal spatial modulating element) 110. Drive elements and wirings are formed on the inner surfaces of the glass substrates 101 and 102, and alignment films subjected to rubbing are coated on the glass substrates.
The rubbing directions are crossed on the two glass substrates as indicated by arrows R1 and R2, and take an angle of 45.degree. relative to the horizontal and vertical directions. A pair of polarizers 116 and 117 are disposed outside of the liquid crystal display element 110. In this example, the polarizing axes P1 and P2 of the polarizers 116 and 117 are parallel (normally-on (normally-white) type) to the rubbing directions R1 and R2 on the adjacent glass substrates 101 and 102. The polarizing axes P1 and P2 may be crossed as indicated at R1 and R2 by broken lines (normally-off (normally black) type).
An illumination optical system 111 including a parabolic mirror and a point light source positioned at the focal point of the parabolic mirror, vertically supplies parallel light fluxes to the polarizer 116 and to liquid crystal display element 110. Light fluxes transmitted through the liquid crystal display element 110 and the polarizer 117 are converged by a field lens 112 and become incident upon a projecting lens 114. The projecting lens 114 enlarges and projects an image formed by the liquid crystal display element 110 upon a screen 119. An optical axis 0 of the projecting lens 114 is eccentric from the centers of the liquid crystal display element 110 and screen 119 in order to adjust the height of the screen 119.
FIG. 9B is a graph showing a relation between a voltage applied to the liquid crystal display element 110 and a transmission of light transmitted through the liquid crystal display element, the angle of light incident upon the liquid crystal display element 110 being used as a parameter. In the example shown in FIG. 9A, the state without an applied voltage to the liquid crystal display element 110 is the light transmission state with a highest transmission coefficient. As the voltage applied to the liquid crystal display element is increased, the transmission gradually lowers and finally becomes 0. The characteristics are indicated by a solid line in the graph of FIG. 9B.
If the light fluxes supplied to the liquid crystal display element 110 by the illumination optical system 111 contain flux components not perpendicular to the substrate surface, the transmission curve shown in FIG. 9B changes from the solid line TO to a broken line T1 or a dot line T2.
If the direction of incident light changes from the horizontal direction shown in FIG. 9A to the lower side, the transmission curve shown in FIG. 9B changes to the broken line T1. In this case, the transmission lowers more rapidly than the perpendicular incidence, and after it takes a local minimum, it rises again and then lowers further.
If the direction of incident light changes from the horizontal direction shown in FIG. 9A to the upper side, the transmission curve shown in FIG. 9B changes to the dot line T2. Namely, as the applied voltage rises, the degree of lowering the transmission lowers, showing as a whole the lifted transmission curve.
If the direction of light fluxes incident upon the liquid crystal display element 110 broadens in an expanded area, the solid line, broken line and dot line transmission curves shown in FIG. 9B are present in a mixed way. Specifically, even if vertical incidence light can be almost shielded, light incident from the lower side or upper side cannot be shielded perfectly. Therefore, in order to provide a high contrast image, the angle of incidence is desired to be maintained vertical to the substrate surface of the liquid crystal display element.
As described above, it is possible to provide a liquid crystal display element having the uniform characteristics by applying incident light vertically to the liquid crystal display element. However, it is not easy to supply only vertical incidence light to the liquid crystal display element. Therefore, the contrast is limited to about 200 at a maximum in practice.
If the direction of incidence expands too broad, the contrast possibly changes on the display surface of a liquid crystal display element. For example, if the contrast cannot be maintained uniform over the whole display surface, undesired patterns such as striped patterns are formed when a black display is to be performed over the whole display surface.